Continuous casting systems are well known in the prior art. In general, liquid metal flows from a reservoir or tundish, through a nozzle and into a continuous casting machine. Various casting machines are known, including twin roller, twin belt, and one belt systems, in which the liquid metal is delivered between the rollers (or belts) and cools and solidifies therein. There is also known single roller strip-casting systems, in which the liquid metal is supplied to the surface of the roller, and cools and solidifies thereon.
In all of the known continuous casting machines, variations in the flow rate of the liquid metal can have a large (and usually detrimental) effect on the quality of the cast metal product. It is therefore important that the rate at which liquid metal is delivered through the nozzle to the casting machine is carefully controlled to be as constant as possible.
Various means have been proposed for ensuring that liquid metal can be delivered to the casting machine at a highly controlled, and substantially constant rate.
For example, U.S. Pat. No. 3,384,150 (Ewsome) discloses a system in which a reservoir of molten metal is located within a pressure vessel. The reservoir is connected to a tundish so that molten metal can be forced, by means of gas pressure within the pressure vessel, from the reservoir to the tundish. In addition, the tundish is enclosed so that gas pressure can be applied to the liquid metal within the tundish. In operation, a quantity of molten metal is supplied to the reservoir, and the pressure vessel is then sealed. At this point, pressurised gas is supplied to the pressure vessel to force the molten metal into the tundish. A further supply of pressurised gas is provided to the tundish, to force the molten metal from the tundish and into a mold or casting machine. The gas pressure in the tundish is controlled to maintain a constant flow rate into the mold, while the pressure in the pressure vessel is varied to maintain a constant level of molten metal in the tundish.
U.S. Pat. No. 4,449,568 (Narasimham) discloses another system in which an inverted pressure bell is partially immersed in liquid metal in a tundish. By varying the gas pressure in the pressure bell, the level of liquid metal outside the pressure bell (and thus the hydrostatic pressure at the tundish outlet) can be maintained substantially constant.
Both of these known systems teach the use of a pressurised gas acting directly on the molten metal as a means for controlling the flow of molten metal from the tundish. However, the system taught by U.S. Pat. No. 3,384,150 relies upon a combination of pressure applied within the tundish, and maintenance of a substantially constant metal level within the tundish, to ensure constant metal flow rate to the mold. The system disclosed in U.S. Pat. No. 4,449,568 relies exclusively on maintenance of a constant level of molten metal in the tundish to ensure a constant rate of flow therefrom.
In either of the above-mentioned prior art systems, if the level of metal in the tundish is allowed to drop, due, for example, to an interruption of the flow of metal into the tundish, maintenance of a constant flow rate of metal into the casting machine would become difficult or impossible. A further disadvantage of the prior art systems is that operating by means of gas pressure acting directly on the liquid metal necessarily complicates the liquid metal handling system, thereby increasing its cost and the risk of failure. Furthermore (particularly in the case of U.S. Pat. No. 3,384,150), true continuous casting is impossible, because a reservoir of molten metal must be placed in a sealed chamber prior to beginning the casting operation. When the liquid metal in the reservoir is consumed, the casting operation must be interrupted to permit the supply of liquid metal in the reservoir to be replenished.
U.S. Pat. No. 4,471,831 (Ray) discloses a continuous casting machine having a chamber which can be pressurized with an inert gas to prevent oxidation of the finished metal product. In order to control the flow of molten metal from the tundish to the nozzle, Ray teaches the use of a shutter or the like mounted at the nozzle outlet.
The above-noted references (and in particular the patents to Narasimham and Ray) teach the art of casting very thin filaments of metal (i.e. a 15 to 100 micron thick sheet or strip) at high speeds. The dimensions of the nozzle orifices used are several hundred times this product dimension (0.06 to 0.1 inch in Ray), and thus do not directly influence the thickness of the finished product. The use of protective gases (as taught by Ray) is known to be necessary to prevent surface degradation of the thin filament. In the field covered by the above references, and known as Rapid Solidification Technology, it is also known to use pressure above the (small) melt to force liquid through a small (see Ray, for example) orifice to form a pendant drop which is maintained in place by surface tension and thus remains stationary, i.e. there is no flow of liquid. The rotating casting wheel is then elevated into contact with this liquid drop, and drags away a thin liquid film which rapidly freezes (10.sup.6 .degree. C. per second in Narasimham) to become the desired glassy metal filament.
There are situations, however, where it is desired to continuously cast a metal product having product thickness dimensions very much larger than those produced by the devices taught by Ray and Narasimham. In such cases, it is often desired that the product thickness be in the range of 1 mm-100 mm (.apprxeq.0.03-4.0 inch), requiring a substantial flow rate of metal through the nozzle while maintaining accurate control of such flow rate, and control of the cross-section of the moving liquid stream. In these situations of sheet, strip or plate casting, it is desired to continuously cast a metal product having controlled dimensions which are substantially determined by the internal shape of the nozzle outlet. It is also imperative that the method used to control the metal flow rate must not alter either the dimensions of the nozzle outlet (as would result from the use of a shutter or the like), or the flow stream of liquid metal as it exits the nozzle outlet.